This invention is a device to monitor the position of an X-ray beam relative to a preferred position.
Since the early days of synchrotron radiation, users have been concerned with the position and stability of the X-ray beam. Third-generation synchrotron sources with undulators pose even greater demands on positioning devices given the increased heat load and distances of up to 55 m between the undulator and the first optical element. The application of current optics to synchrotron radiation can produce uncollimated beam dimensions at the sample of 38 xcexcm, vertical, by 83 xcexcm, horizontal for a beam having a photon flux density of 3xc3x971015 photons sxe2x88x921 mmxe2x88x922 at 12 keV and 100 mA. The high flux density and low convergence of the focused beam permit further reductions in beam size through collimation while retaining sufficient flux for fast data collection. These characteristics are useful for investigating small samples. With collimating slit dimensions of 50 xcexcmxc3x9750 xcexcm or less, it is necessary to have precise monochromatic beam positioning. When the beam size reaches 1-2 mm a quadrant PIN photodiode can be used to intercept the edges of the focused beam while passing the main beam undisturbed. Current density limits of the photodiode prevent detection of the full undulator beam. In addition, the outer edges of the X-ray beam may not accurately describe the center-of-mass of the focused beam, leading to positioning errors. For narrow-beam applications, the method of choice has been the split ion chamber. The split ion chamber is essentially xe2x80x98non-interferingxe2x80x99, is capable of less than 5 xcexcm positioning accuracy, and has a linear working range of a few millimeters. Split ion chambers can, however, suffer from recombination and space charge effects and can report false readings when operated near a strong scattering source. In addition, the device is not vacuum compatible and must be relatively long, typically xe2x89xa75 cm in length to achieve adequate signal strength. Since split ion chambers measure in only one dimension, two such devices are required to obtain horizontal and vertical information simultaneously.
An alternative to the split ion chamber has been developed for small beams using a thin CVD diamond photodetector. This device demonstrates relatively high position sensitivity for low energy X-rays e.g. 2-10 keV. However, position resolution is currently beam-size dependent with the best resolution obtained for beam sizes close to half the inter-electrode spacing of 0.5 mm. Errors in determining the beam centroid may also occur since only the outer portion of the beam is used for position determination.
The subject invention relates to an improved device and method for monitoring the position of an X-ray beam using four PIN photodiode detectors and fluorescence radiation generated from a metal foil target placed in the path of the X-ray beam. The device is vacuum compatible and can operate over a wide energy range. The primary purpose of the beam-position monitor will be to correct tune losses when changing the energy of the monochromator and to measure changes in beam position during data collection.
Thus, one object of this invention is to provide a position-sensitive PIN diode array apparatus to detect monochromatic beam movement in both the vertical and horizontal direction.
Another objective of this invention is the capability of optimizing the metal foil target selected for the correct absorption, thickness and fluorescence to correspond with the energy of the beam.
Another object of the invention is the capability to provide for a number of different metal foils within the vacuum chamber to provide for the ability to change the metal foil employed without breaking the vacuum or disrupting the beam.
Another object of the invention is to provide a mechanism whereby the position of the PIN diodes can be altered during calibration without disrupting the beam.
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.
To achieve the foregoing and other advantages, this invention is an apparatus and method for determining the position of an X-ray beam. This information is needed in order to keep the X-ray beam focused on the pre-sample slits. This is accomplished by use of a four PIN diode array located upstream of a metal foil which is positioned so as to intersect the x-ray beam. The metal foil has an approximate thickness of 0.5 xcexcm and where the foil is positioned in the X-ray beam. Using conventional difference-over-the-sum techniques, two-dimensional position information is obtained from the metal foil fluorescence. Because the full X-ray beam passes through the metal foil, the true center of the beam is measured. Since the PIN diodes are positioned away from the direct X-ray beam, diode linearity is maintained at all undulator power levels. After calibration, the apparatus provides magnitude and direction of a change in beam position, making it possible to compute exact position corrections. The device combines high sensitivity with wide spacing between diodes, thus, allowing the array to be used for coarse adjustments as might be encountered during beamline reconfiguration. The diode spacing can be altered to increase diode sensitivity or expand the operating displacement range. In addition, the foil thickness, absorption, and fluorescent radiation can be altered to provide for high energy beams.
In addition, the apparatus can be placed after the beam defining slits to measure the intensity of the beam. Accurate monitoring of the X-ray beam incident to the sample makes it possible to scale experimental data with any changes in the incident X-ray flux.